Fluid fuel burner control with purge

ABSTRACT

A thermostatically operated fluid fuel burner control system in which operation of an initiating relay is effected upon a demand for heat. The relay energizes a thermal safety time switch, including an electrical resistance heater, through a pair of normally closed contacts of a flame responsive switch. The thermal switch, is delayed in operation to provide a purge period. Upon operation the thermal switch opens its energizing circuit and completes an energizing circuit for fuel feeding mechanism and a spark generator, the later circuits shunting the normally closed flame switch contacts. Both these sets of thermal switch contacts latch in operated condition. Thus, during the &#39;&#39;&#39;&#39;cool down&#39;&#39;&#39;&#39; period of the warp switch heater, these contacts maintain the spark generator and fuel feeding mechanism energized to attempt ignition of the fuel. Upon failure of ignition to occur during this timed trial period, the system goes to &#39;&#39;&#39;&#39;lock out,&#39;&#39;&#39;&#39; requiring release of the relay by manual actuation of the thermostat to interrupt the power circuit to resume operation. The relay is mechanically interlocked with the latched warp switch contacts, and upon release, cams them into their initial condition for a new attempt at ignition. After a successful ignition, the warp switch, upon cooling, actuates a pair of contacts to stop operation of the spark generator, while maintaining the fuel feeding means in operation. Any temporary power interruption to the control circuit causes the initiating relay to release, interrupting feeding of fuel, while the mechanical interlock cams the previously mentioned warp switch contacts to their initial condition such that reoperation of the relay for a new trail for ignition must first await cool down of the flame responsive switch, which is of the time delay type (such as a thermocouple).

United States Patent [721 Inventor Gerald E. Dietz Milwaukee. Wis. [21 1 Appl. No. 873,245 [22] Filed Nov. 3, 1969 [45] Patented June l, 1971 [73] Assignee Penn Controls, Inc.

Oak Brook, Ill.

[54] FLUID FUEL BURNER CONTROL WITH PURGE Primary Examiner-Carroll B. Dority, Jr. Attorney-A. J. De Angelis ABSTRACT: A thermostatically operated fluid fuel burner control system in which operation of an initiating relay is effected upon a demand for heat. The relay energizes a thermal 56 FL TC TCR I M Q thrash,

safety time switch, including an electrical resistance heater, through a pair of normally closed contacts of a flame responsive switch. The thermal switch, is delayed in operation to provide a purge period. Upon operation the thermal switch opens its energizing circuit and completes an energizing circuit for fuel feeding mechanism and a spark generator, the later circuits shunting the normally closed flame switch contacts. Both these sets of thermal switch contacts latch in operated condition. Thus, during the cool down" period of the warp switch heater, these contacts maintain the spark generator and fuel feeding mechanism energized to attempt ignition of the fuel. Upon failure of ignition to occur during this timed trial period, the system goes to lock out," requiring release of the relay by manual actuation of the thermostat to interrupt the power circuit to resume operation, The relay is mechanically interlocked with the latched warp switch contacts, and upon release, cams them into their initial condition for a new attempt at ignition- After a successful ignition, the warp switch, upon cooling, actuates a pair of contacts to stop operation of the spark generator, while maintaining the fuel feeding means in operation. Any temporary power interruption to the control circuit causes the initiating relay to release, interrupting feeding of fuel, while the mechanical interlock cams the previously mentioned warp switch contacts to their initial condition such that reoperation of the relay for a new trail for ignition must first await cool down of the flame responsive switch, which is of the time delay type (such as a thermocouple).

PATENTEB JUN, '1 l97| SHEET 1 UF 2 ATTORNEY! GERALD E. DIETZ INVENTORZ FLUID FUEL BURNER CONTROL WITH PURGE The invention relates to automatic control system for fluid fuel burners, which system include safety means operative to permit the continued supply of fuel to the burner when there is combustion and to stop the flow of fuel should combustion fail to occur within a predetermined time interval during which the fuel is supplied, or, if combustion has been established, subsequently fails. More particularly, the invention relates to such systems which provide a timed delay between cycles of operation to allow purging of unburned fuel.

In present day fuel burner controls for heating installations, usually safety means control energization of fuel feeding mechanism in response to a thermostat located in the space being heated. The safety means usually include flame detection means and a timer for timing the interval during which fuel is supplied and at the expiration which interval, if combustion has not occurred, the burner system is shut down, the interval normally being termed the trial for ignition period. These controls must be of utmost reliability and quick acting in order to prevent hazardous accumulation of unburned fuel in the burners.

It is desirable to provide a time delay between cycles of operation of the burner mechanism in order to exhaust accumulated unburned fuels either by power vent means, or through natural gravity flow.

It is, therefore, an object of the invention to'provide a fuel burner control which is of maximum reliability, economical to manufacture and maintain, and which provides a period between cycles of operation to purge the burner of unburned fuel.

In carrying out the invention in accordance'with a preferred embodiment, normally closed contacts of a thermocouple actuated switch (flame responsive switch) are provided in the energizing circuit of a control relay which is subject to a thermostat locatedjn the space being heated. Normally open contacts of the control relay are provided in series with a warp switch heater and normally closed flame switch and warp switch contacts, while a pair of normally open warp switch contacts shunt the flame switch contacts. Operation of the relay causes energization of the warp switch heater through the normally closed flame and warp switch contacts. The warp switch, upon operation, deenergizes the heater circuit starting a timed cool down period and provides a holding circuit across the flame switch contacts during the trial for ignition, the latter warp switch contacts latching in such condition. The warp switch, upon operation, also energizes fuel feeding means and initiates operation of a spark generator to attempt to ignite the fuel..lf ignition does not occur during the cool down" period of the warp switch, when the latter releases, feeding of fuel and spark generation is terminated, while the control relay remains operated. The latched open condition of the warp switch contacts in the energizing circuit of the warp switch heater prevents reoperation for a second ignition trial. The system is, therefore, on lock out." For a second trial, the thermostat contacts must be manually opened to release the relay which, through a mechanical interlock, earns the latched warp switch contacts to their initial condition. Under conditions where during trial for ignition, ignition occurs, the flame switch contacts (shunting the warp switch contacts) open, while a second pair of flame switch contacts closes, completing a holding circuit through the latched closed warp switch contacts for the fuel feeding means. This circuit maintains the fuel feeding means energized after warp switch contacts, upon cool down of the warp switch, terminate spark generation.

Any interruption of power to control circuit during operation causes immediate release of the control relay. This relay through a mechanical interlock earns the aforementioned holding circuit warp switch contacts open, preventing reoperation of the sparking mechanism until the thermocouple has cooled sufficiently to release the flame switch, thus, reclosing the flame switch contacts in the energizing circuit of the relay.

To recapitulate, there is provided a control with trial for ignition" during the warp switch cool down" which is, thus, independent of line voltage variations. The control provides an automatic purge period delaying recognition attempts between cycles of operation to enable the exhausting of unburned hazardous fuel with lock out of the system upon a failure to ignite during any attempted ignition. Through a simple circuit utilizing mechanical interlock of a relay and warp switch, a safe, reliable, inexpensive and easy to maintain control circuit which is extremely safe is thus provided. Failure of the relay contacts, thermocouple switch contacts or warp switch contacts to close in the subject circuit, as for example, by dirt inadvertently getting caught between the contacts, causes the system to go to lock out providing an extremely safe control circuit.

Features and advantages of the invention will be seen from the above, from the following description of operation of the.

preferred embodiment when considered in conjunction with the drawings and from the appended claims.

In the drawings:

FIG. 1 is a simplified, schematic wiring diagram of a fuel burner control system including a diagrammatic representation of a gas main fuel burner, thermocouple, igniter and a main gas valve, embodying the invention;

FIG. 2 is a diagrammatic representation in perspective of a warp switch timer and control relay combination with mechanical interlock between the relay and switch as used in the circuit of FIG. 1;

FIG. 3 is a side view of the relay switch combination of FIG. 2 with portions omitted for clarity and showing both the relay and switch in actuated positions;

FIG. 4 is a view similar to that of FIG. 3 but with the warp switch in its cooled down" condition; and

FIG. 5 is a fragmentary partial view of a portion of FIG. 4 showing the relay in released condition having cammed latched warp switch contacts back to their initial condition.

With reference to FIC-v l, alternating power from any convenient source (not shown) is applied to the circuitry over supply lines L1, L2.

A step down transformer TRS applies approximately 25 volts of alternating power through thermostat contacts T to the control circuit. For simplicity, the circuit is shown for controlling a gas burner, comprising a main gas valve MGV controlled by a solenoid SEL for feeding gas in the direction of the arrows to a burner BR that may be ignited by sparks across a spark gap SG; ignited fuel being indicated as a flame FL. A thermocouple TC positioned to be heated by the burning fuel FL is connected across the coil TCR of a thermocouplerelay. When thermocouple TC is heated, sufficient current is generated to energize the relay TCR.

Sparks for igniting the fuel are provided across spark gap SG by a spark generating circuit comprising an ignition transformer IGN, a silicon controlled rectifier SCR, and its firing circuit comprising resistor R, capacitor C and diode D. This spark generating circuitis described and shown, in U.S. Pat.

No. 3,457,456 issued to the inventor G. Dietz, on July 22,.

A warp switch for timing the trial for ignition period includes a warp switch heater WSH. A control relay coil designated RL is connected across the secondary of transformer TRS through thermostat T and normally closed contacts TCRl of thermocouple relay TCR. A pair of normally.

open contacts RLI are. placed in series with warp switch heater WSH and a pair of its normally closed interlock contacts WSl. A pair of normally open' warp switch contacts WS2 shunt contacts TCRI. The solenoid SEL of the main gas valve and the spark generating circuit are connected across the secondary of transformer TRS through thermostat contacts T, relay contact RLI and a pair of normally open warp switchcontacts W83. A circuit shunts contacts W83 through normally open thermocouple relay contacts TCR2 and normally open warp switch contacts WS2.

A pair of normally closed warp switch contacts WS4 are provided across the gate-cathode circuit of silicon controlled rectifier SCR to prevent spark; generation under cooled conditions of the warp switch.

A mechanical interlock is provided between relay RL and warp switch contacts WSl, WS2 as is indicated by broken line BL and as will now be described in detail with reference to the warp switch and relay structure shown in FIG. 2 through 5.

A control relay, generally designated 10, and the warp switch, generally designated 12, are mounted on a printed circuit board 14 in any convenient manner such as by screws 16. The designations used for relay and warp switch components appearing in the circuit of FIG. i are also used in FIGS. 2 through 5. Relay includes a relay coil RL mounted on a magnetic frame 18 with an armature 20 hinged on a knife edge at 22 for magnetic attraction, when the coil is energized. to the pole face of a pole 24 (FIG. 3) around which relay coil RL is wound. Armature 20 is biased away from pole 24 for a coil spring 26 under tension in the usual manner. Armature 20 carries a movable contact 30 for cooperation with a stationary contact 32, the latter being mounted on board 14 and the pair forming a normally open contact pair, designated RLI. Relay coil RL is provided with two interconnecting wires 34 and 36 for connection to external circuitry as shown in HO. 1.

Warp switch 12 includes a warp switch heater WSH insu iatedly wound around a bimetal 40 cantilevered from circuit board 14. Bimetal 40 is provided at its free end with an actuator 42 adjustable positioned on bimetal 40 by an adjusting screw 44. The warp switch is provided with a pile up assembly, generally designated 46 of stationary and movable leaf spring contacts insulated from each other by insulators 48 in stacked relation and in the usual manner cantilevered from board 14. The "pile up stacked arrangement includes two movable leaf springs 50 and 52. Movable leaf spring 50 is provided with contact 54 (FIG. 3) on one side and a contact 55 on the other, while movable leaf spring 52 is provided with a contact 56 on one side (FIG. 3) and contact 57 on the opposite side.

On stationary leaf spring 58, a contact 60 is positioned for cooperation with movable contact 54 of leaf spring 50 to provide a contact pair WSl. Stationary leaf spring 62 is equipped with a contact 64 mounted for cooperation with movable contact 55 to provide warp switch contact pair WS2. A third stationary contact leaf spring 68 with a contact 70 is positioned for cooperation with movable contact 56 of movable ieaf spring 52 to provide warp switch contact pair WS4. Movable contact 57 cooperates with an adjustable mounted stationary contact 71 in the form of an adjustable screw threaded through a stationary cantilever 72 to provide warp switch contact pair W83.

Toggle springs 74 and 76 are provided for movable leaf springs 50 and 52 respectively, to provide snap actuation of the leaf springs. Toggle spring 76 is mounted with one end engaging the free end of leaf spring 52 and its other end engaging a central pip 80 (FIG. 2) of a split resilient leaf spring 82 fastened to actuator 42 by means of cotter pin 84. Similarly, one end of toggle spring 74 engages movable leaf spring 50, while at its other end is mounted on a split leaf spring 90. An adjustable screw 92 is threaded through the free end of split spring 90 to engage the fastened end of split leaf spring 82 for actuation by warp switch actuator 42.

A lever 100 (FIG. 2) is pivotable mounted at 102 to a member 104 protruding from the top of the stacked leaf springs and insulators 46 of warp switch 12. Pivoted lever 100 has one arm 100A extending in position to engage split leaf spring 90 of warp switch contact pairs WSl and WS2 (FIG. 3). Its other arm 100B extends in position for coaction with armature 20 of relay 10. An adjustable screw 10% is threaded through a portion of lever arm 1008 in position for engagement with armature 20.

Lever 100 provides a mechanical interlock between armature 20 and split leaf spring 90 for actuation of warp switch contacts WS1 and WS2, as will be later described.

In operation, upon sufficient energization of reiay coil RL of relay 10, armature 20 is attracted towards the face of pole 24, as shown in FIG. 3, closing relay contacts RLl. Upon sufficient energization, warp switch heater WSH heats bimetal 40 sufficiently to actuate the bimetal towards the right in FIG. 3. causing snap action actuation by means of toggle springs of warp switch contacts WS4 open and WS3 closed by means of split leaf spring 82 and toggle spring 76 and snap actuation of contacts WSl open and contacts WS2 closed by means of toggle spring 74 and split leaf spring as shown in FIG. 3.

Contacts WSl and WS2 are constructed to remain latched in this actuated condition after cool down" of the warp switch i.e. after warp switch heater WSH has been deenergized causing bimetal 40 to return to its initial unactuated condition (FIG. 2).

Next assume this cooled down condition of the warp switch after actuation, as is shown in PEG. 4. Under such conditions warp switch contacts WS4 return to closed condition while warp switch contacts WS3 return to their normally open condition. However, warp switch contacts WSI and WS2 remain latched in their actuated condition shown, that is, with con tacts WSl open and contacts WS2 closed. These contacts remain in latched condition until relay l0 releases.

Next assume that relay 10 releases under conditions where its coil RL is deenergized. Under such conditions, armature 20 under the influence of its biasing spring 26 (FIG. 2) moves away from pole 24, as shown in FIG. 5, separating relay contacts RLl. Armature 20, in moving away from the pole, engages adjustable screw 103 carried by lever 100. This pivots lever l.-0 about its pivot 102 counterclockwise (FIG. 5) causing its arm A to engage split leaf spring 96, camming warp switch contacts WS2, WSI in snap fashion back to their initial condition, as shown, with contact WSl reclosed and WS2 reopened and as initially described with relation to FIG. 2. Thus, upon deenergization of coil RL and release of relay 10, the mechanical interlock between armature 20 and warp switch 12 by means of lever 100 cams warp switch contacts WSi to reclose and warp switch contacts WS2 to reopen from their latched condition, for purposes which will now be described.

In operation, assume that with power supplied to the circuitry of FIG. 1, thermostat T engages its contacts calling for heat. This completes an energizing circuit for control relay RL through normally ciosed thermocouple (flame) switch, contacts TCRI. Upon operation, relay RL engages its contacts RLl, completing an energizing circuit for the heater WSH of the warp switch through its normally closed contacts WSi and flame switch contacts TCRI.

It may be noted that relay RL in certain embodiments may also operate a second pair of contacts to energize auxiliary mechanism such as a blower motor to purge the burner chamber of fuel, while contacts RLl may be of an air switch responsive to movement of the purging air by the blower motor.

After sufiicient heating of the warp switch heater WSH, the warp switch operates, opening its interlock contacts WSl in its energizing circuit, thereby terminating energization of the heater and closing its contacts WS2 shunting thermocouple switch contacts TCRl, but without effect at this time. As was previously described, these warp switch contacts WS1 and WS2 latch in this actuated position and remain therein after cool down of the warp switch bimetal and its return to initial condition.

Operation of the warp switch also causes its contacts W84 to open, removing the shunt across the gate-cathode circuit of silicon controlled rectifier SCR, preparing the spark generating circuit for operation to provide a spark across spark gap SG to ignite the fuel at burner BR.

Simultaneously, warp switch contacts W53 close completely the spark generating circuit and a circuit for solenoid SEL through relay contacts RL] and thermostat contacts T, presently closed. The solenoid SEL of main gas valve MGV, upon operation, feeds fuel to burner BR for ignition by the sparks at gap SG.

Upon deenergization of heater WSH, the warpv switch heater starts its cool down" period during which the fuel fed to the burner must be ignited to maintainsystem operation.

Assume that the heater WSH cools sufficiently to cause the warp switch to release before the fuel is ignited. Under such conditions, the warp switch recloses its contacts WS4, terminating spark generation, and opens its contacts W53, interrupting the circuit to solenoid SEL closing the gas valve MGV stopping fuel flow to burner BR. Warp switch contacts W51, W52, as has been described, remain latched in opened and closed position, respectively. Thus, latched open warp switch contacts WSl prevent reenergization of heater WSH, placing the system on lock out until relay RL is caused to release and cam the latched warp switch contacts WSl, WS2 to their original position. Relay RL. may be released by manual actuation of thermostat contact T to open position.

Next assume that ignition of the fuel takes place before .the warp switch heater has cooled down sufficiently to release. Under such conditions, flame FL starts to heat thermocouple TC. When thermocouple TC has been heated sufficiently it generates enough current to operate thermocouple relay TCR. Relay TCR closes its contacts TCR2, completing a holding circuit for the main gas valve solenoid SEL through latched closed warp switch contacts WS2. Thermocouple relay also opens its contacts TCR] in the energization circuit of relay RL, but without effect, since warp switch contacts WS2 remain latched closed, as was previously stated, to maintain the relay energized.

Next assume that the warp switch has cooled sufficiently to release and return to its initial condition. The warp switch recloses its contacts WS4, replacing a short circuit across the gate-cathode of the silicon controlled rectifier SCR thereby terminating spark generation at spark gap 86. The warp switch also reopens its contacts WS3 in the initial operating circuit for the main gas valve solenoid SEL but without effect, since this circuit is maintained through now closed thermocouple contacts;TCR2 and latched warp switch contacts WS2. Therefore, with fuel ignited as sensed by flame switch TCR, the system remains in operation after the release of the timing warp switch.

Next assume that power interruption to the circuit occurs either through the opening of thermostat contacts T at the satisfaction of the demand for heat or through momentary power interruption as sometimes happens inadvertently. Under such conditions, relay RL releases, opening its contacts RLl, interrupting the holding circuit through gas valve solenoid SEL. Main gas valve solenoid SEL releases, closing the flow of gas through valve MGV to the burner, terminating ignition of the fuel and starting cool down" of thermocouple Relay RL in releasing, also cams latched warp switch contacts WSl to reclose and W82 to reopen, as was previously described, preparing an energizing circuit for warp switch heater WSH through contacts TCRl after the thermocouple has cooled sufficiently for subsequent reignition. Relay R'L in camming the previously latched warp switch contacts WSI and WSZ to their original position places the ignition system subject to cool down of thermocouple TC before a second attempt to reignite can be initiated. This provides a purge period during which unburned fuel maybe exhausted by natural gravity flow, in addition to the previously described prepurge period upon reoperation of the relay RL and during reenergization of warp switch heater WSH.

Next assume that thermocouple TC has cooled sufficiently to cause thermocouple relay TCR to release. Relay TCR reopens its contacts TCRZ but without effect, and also recloses its contacts TCR preparing an energizing circuit for controlled relay RL through thermostat T. Should thermostat T reclose and power be reapplied, the previously described cycle of operation is resumed with energization of relay RL and warp switch heater WSH and a trial for ignition during cool down of warp switch heater WSH.

It may be noted that should dirt inadvertently maintain contacts TCRl from closing, the system would go to lock out."

The same is true for dirt in contacts W81, W82 or W53.

Thus, the subject arrangement provides an extremely safe, simple and economical to manufacture and maintain fuel burner control through the utilization of a warp switch and relay combination with mechanical interlock of the relayarmature with warp switch latched contacts to provide a purge period between cycles of operation, while a flame responsive thermocouple switch cools down and provides an automatic lock out" condition, upon failure to ignite the fuel during a timed trial period.

As changes can be made in the above described construction and many apparently different embodiments of this invention can be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown on the accompanying drawing be interpreted as illustrative only and not in a limiting sense.

'What I claim is:

1. An electrical control for the operation of a fluid fuel burner comprising,

a source of power,

fuel feeding means selectively energizable from said power source,

fuel igniter means also selectively energizable from said I power source,

control relay means including an energization coil and being operative for controlling energization of said fuel feeding means and said igniting means,

thermostat means responsive to the ambient temperature in a space to be heated by said burner and being connected in series circuit with said coil of said relay means for controlling energization thereof,

a flame responsive switch positioned to respond to flame at said burner,

an electrothermal delay means for timing a trial for ignition period,

said flame responsive switch in nonresponsive condition preparing an energizing circuit for said control relay means and said electrothermal delay means.

said control relay means upon operation energizing said electrothermal delay means initiating timing for a purge period,

said delay means a predetermined interval after energization operating into a condition latching its energization circuit open, terminating said purge period while providing an energizing circuit for said igniter means and said fuel feeding means for operation of both to ignite fuel fed to said burner,

said electrothermal time delay means, upon operation, initiating a timing period for igniting said fuel at the expiration of which time said time delay means terminates operation of said igniter means and said fuel feeding means under conditions where said flame responsive switch remains in nonresponsive condition, and

under conditions where said flame responsive switch is actuated, said flame responsive switch providing a holding circuit for said fuel feeding means, characterized in that,

said electrothermal delay means includes circuit means which remain latched in operated condition once operated, for providing a holding circuit for said control relay means, while preventing reoperation of said electrothermal delay means, and

wherein amechanical interlock is provided between said control relay means and said latched circuit means of said electrothermal time delay means for camming said latched delay circuit means to their original condition under conditions where said control relay releases permitting reoperation of said systems only under conditions where said control relay means first is restored to unoperated condition.

2. An electrical control for the operation of a fluid fuel burner comprising,

a source of power,

fuel feeding means selectively energizable from said power source,

fuel igniter means also selectively energizable from said power source, r

control relay means including an energizing coil and being operative for controlling energization of said fuel feeding means and said igniting means,

thermostat means responsive to the ambient temperature in a space to be heated by said burner and being connected in series circuit with said coil of said relay means for controlling energization thereof,

a flame responsive switch positioned to respond to flame at said burner, and

an electrothermal safety switch including a heater, :1 first normally closed contact pair, a first normally open contact pair, a second normally closed contact pair and a second normally open contact pair,

said flame responsive switch including a pair of normally closed contacts in series circuit with the coil of said control relay and a second pair of normally open contacts connected for providing a holding circuit for said fuel feeding means and said fuel igniter means,

said control relay upon operation, completing an energizing circuit for said heater of said electrothermal switch through its said said first set of normally closed contacts and said first normally closed set of contacts of flame responsive switch for timing a purge period,

said electrothermal switch upon sufficient heating of its said heater operating to latch open its first set of normally closed contacts interrupting its energizing circuit completing said purge period, and latch closed its first set of normally open contacts shunting said first set of normally closed flame responsive switch contacts said electrothermal switch upon operation also engaging its said second pair of normally open contacts providing an energizing circuit for said fuel feeding means for operation thereof to fuel said burner and opening its said second pair of normally closed contacts for initiating operation of said igniter means to provide an igniting spark for said fuel,

said electrothermal switch upon sufflcient cool down after said deenergization reclosing its said second pair of normally closed contacts stopping operation of said igniter means and reopening its said second pair of normally open contacts transferring said fuel feeding means to said holding circuit subject to said normally opened contacts of said flame responsive switch,

characterized in that said electrothermal switch is constructed to latch its said first normally closed set of switch contacts and its said first normally open set of switch contacts in their actuated position,

and wherein said control relay is mechanically interlocked with said electrothermal switch to cam both said latched electrothermal switch contact pairs to their original position under conditions where said relay releases.

3. An electrical control as set forth in claim 2 wherein,

said electrothermal switch comprises a warp switch.

4. An electrical control as set forth in claim 2 wherein,

said flame responsive switch includes a thermocouple positioned for heating by said burner, and

a thermocouple relay energized by current generated by said thermocouple for operation when said thermocouple has been heated sufficiently. 

1. An electrical control for the operation of a fluid fuel burner comprising, a source of power, fuel feeding means selectively energizable from said power source, fuel igniter means also selectively energizable from said power source, control relay means including an energization coil and being operative for controlling energization of said fuel feeding means and said igniting means, thermostat means responsive to the ambient temperature in a space to be heated by said burner and being connected in series circuit with said coil of said relay means for controlling energization thereof, a flame responsive switch positioned to respond to flame at said burner, an electrothermal delay means for timing a trial for ignition period, said flame responsive switch in nonresponsive condition preparing an energizing circuit for said control relay means and said electrothermal delay means. said control relay means upon operation energizing said electrothermal delay means initiating timing for a purge period, said delay means a predetermined interval after energization operating into a condition latching its energization circuit open, terminating said purge period while providing an energizing circuit for said igniter means and said fuel feeding means for operation of both to ignite fuel fed to said burner, said electrothermal time delay means, upon operation, initiating a timing period for igniting said fuel at the expiration of which time said time delay means terminates operation of said igniter means and said fuel feeding means under conditions where said flame responsive switch remains in nonresponsive condition, and under conditions where said flame responsive switch is actuated, said flame responsive switch providing a holding circuit for said fuel feeding means, characterized in that, said electrothermal delay means includes circuit means which remain latched in operated condition once operated, for providing a holding circuit for said control relay means, while preventing reoperation of said electrothermal delay means, and wherein a mechanical interlock is provided between said control relay means and said latched circuit means of said electrothermal time delay means for camming said latched delay circuit means to their original condition under conditions where said control relay releases permitting reoperation of said systems only under conditions where said control relay means first is restored to unoperated condition.
 2. An electrical control for the operation of a fluid fuel burner comprising, a source of power, fuel feeding means selectively energizable from said power source, fuel igniter means also selectively energizable from said power source, control relay means including an energizing coil and being operative for controlling energization of said fuel feeding means and said igniting means, thermostat means responsive to the ambient temperature in a space to be heated by said burner and being connected in series circuit with said coil of said relay means for controlling energization thereof, a flame responsive switch positioned to respond to flame at said burner, and an electrothermal safety switch including a heater, a first normally closed contact pair, a first normally open contact pair, a second normally closed contact pair and a second normally open contact pair, said flame responsive switch including a pair of normally closed contacts in series circuit with the coil of said control relay and a second pair of normally open contacts connected for providing a holding circuit for said fuel feeding means and said fuel igniter means, said control relay upon operation, completing an energizing circuit for said heater of said electrothermal switch through its said said first set of normally closed contacts and said first normally closed set of contacts of flame responsive switch for timing a purge period, said electrothermal switch upon sufficient heating of its said heater operating to latch open its first set of normally closed contacts interrupting its energizing circuit completing said purge period, and latch closed its first set of normally open contacts shunting said first set of normally closed flame responsive switch contacts. said electrothermal switch upon operation also engaging its said second pair of normally open contacts providing an energizing circuit for said fuel feeding means for operation thereof to fuel said burner and opening its said second pair of normally closed contacts for initiating operation of said igniter means to provide an igniting spark for said fuel, said electrothermal switch upon sufficient cool down after said deenergization reclosing its said second pair of normally closed contacts stopping operation of said igniter means and reopening its said second pair of normally open contacts transferring said fuel feeding means to said holding circuit subject to said normally opened contacts of said flame responsive switch, characterized in that said electrothermal switch is constructed to latch its said first normally closed set of switch contacts and its said first normally open set of switch contacts in their actuated position, and wherein said control relay is mechanically interlocked with said electrothermal switch to cam both said latched electrothermal switch contact pairs to their original position under conditions where said relay releases.
 3. An electrical control as set forth in claim 2 wherein, said electrothermal switch comprises a warp switch.
 4. An electrical control as set forth in claim 2 wherein, said flame responsive switch includes a thermocouple positioned for heating by said burner, and a thermocouple relay energized by current generated by said thermocouple for operation when said thermocouple has been heated sufficiently. 